1. Field of the Invention
The present invention relates to a switching synchronous apparatus for a four wheel drive vehicle, and particularly to a switching synchronous apparatus for a four wheel drive vehicle for switching a state between a two wheel drive state and a four wheel drive state by intermittently attaching and detaching a drive force transmission path of a driven side differential gear.
2. Description of the Related Art
Conventionally, as a driven side differential gear for performing a switching control between a two wheel drive state and a four wheel drive state by intermittently attaching and detaching a drive force transmission to both right and left wheels in accordance with intermittently attaching and detaching a part of a drive force transmission path of the driven side differential gear, there has been known a free running differential gear as shown in Japanese Patent Application Examined Publication No. 5-54574.
FIG. 1 shows a structure of such a free running differential gear, in which a drive force from an engine (not shown) is transmitted to a ring gear 16 meshed with a drive pinion 15 in a bevel gear manner, and transmitted to an outer differential case 1 integrally fixing the ring gear 16, thereby rotating the outer differential case 1. In the middle of the outer differential case 1, a dog clutch 11 an attachment and detachment of which is controlled by a shift fork 18 of a lock cylinder 14 disposed outside a differential carrier 17 fixed to a vehicle body side is provided. And it is structured such that a transmission of a drive force to an inner differential case 6 receiving a pair of left and right side gears 4 and 5 meshed with both sides of a plurality of pinion gears 3, through the pinion gears 3 supported in rotatable to a pinion shaft 2 constituting a normal differential gear, is intermittent by the dog clutch 11. In accordance with the structure mentioned above, a switching control between a two wheel drive state and a four wheel drive state of the type, for example, mainly driving rear wheels is performed by intermittently attaching and detaching a drive force to the inner differential case 6 corresponding to the front wheel side.
Further, as a driven side differential gear, there has been known a structure for performing a switching control between a two wheel drive state and a four wheel drive state by intermittently attaching and detaching one of drive axles connected to the left and right wheels of the driven side.
In this case, a four wheel drive vehicle having a driven side differential gear for performing a switching control between a two wheel drive state and a four wheel drive state by intermittently attaching and detaching a power transmission to both right and left wheels in accordance with intermittently attaching and detaching a part of the power transmission path mentioned above is normally separately provided with a transfer, and is structured such that a drive force is transmitted by a connection of the transfer. For example, in the four wheel drive vehicle performing a switching control between a two wheel drive state and a four wheel drive state of the type mainly driving the rear wheels, a drive force is also transmitted to the front wheel side having the driven side differential gear in addition to the usual rear wheel drive state.
Then, in the case of the example of the free running differential gear shown in FIG. 1, it is structured such that when the transfer is connected, the dog clutch 11 in the free running differential gear is also connected in an interlocking manner. In this case, during the two wheel drive state in which the transfer is not connected, a power transmission system from the transfer to the outer differential case 1 through the drive pinion 15 and the ring gear 16 is at a standstill, so that an energy loss is prevented.
That is, in general, regarding the driven side differential gear mentioned above, the switching between the two wheel drive state and the four wheel drive state is correspondingly performed by intermittently attaching and detaching the transfer.
Here, with respect to the switching between the two wheel drive state and the four wheel drive state, a need for increasing a freedom such that a switching can be performed even when a vehicle runs at about 80 km/h has become increased in accordance with a recent popularization of a four wheel drive vehicle.
At a time of switching a state between the two wheel drive state and the four wheel drive state while the vehicle runs in the above manner, particularly switching a state from the two wheel drive state to the four wheel drive state, for example, in the case of the four wheel drive vehicle performing the switching control between the two wheel drive state and the four wheel drive state of the type mainly driving the rear wheels, it is necessary to connect the transfer to the front wheel side at which a drive is stopped so as to branch the drive force from the rear wheel side drive system at which a drive is performed.
Then, a synchronous mechanism is generally provided in the transfer in order to make it possible to connect the transfer during a running of the vehicle. Specifically speaking, the drive pinion 15, the ring gear 16 and the outer differential case 1 corresponding to the side at which the drive force is drawn out in the driven side differential gear in FIG. 1 are synchronously connected to the drive system by the synchronous mechanism of the transfer, and the dog clutch 11 is in a connection state in accordance therewith, so that the drive force can be smoothly connected to the front wheel side through the pinion gear 3 and the inner differential case 6.
However, there still exist vehicles in which the synchronous mechanism is not provided in the transfer.
Further, even on the assumption that the synchronous mechanism is provided in the transfer, in the vehicle mounting an engine generating greater torque in accordance with a progress in a recent engine control technology and the like, since there is a necessity of increasing a synchronous volume of the synchronous mechanism in the transfer, it becomes a great problem at designing in the transfer having a very narrow layout space for placing the elements.
Accordingly, it is currently desired to provide a switching synchronous apparatus for a four wheel drive vehicle which can be sufficiently applied to a vehicle having no synchronous mechanism in the transfer, inherently gives no influence to a structure of the transfer itself, inherently gives no obstacle to a security of a freedom at designing, and can smoothly switch a state between a two wheel drive state and a four wheel drive state, particularly can smoothly switch a state from a two wheel drive state to a four wheel drive state while the vehicle runs.